1. Field of the Invention
The present invention relates to an in-vehicle pulse radar device for measuring a distance to a neighboring vehicle, a relative speed thereof, etc. for the purposes of car-to-car distance control, collision damage mitigation, and the like.
2. Description of the Related Art
Conventionally, for such in-vehicle pulse radar devices, a millimeter wave in the 76 GHz band is used as a transmission wave. In a case of using such a high frequency wave, there is a problem in that since isolation is hardly secured between elements or lines in general, the transmission pulse wave is directly superimposed on a receiving circuit. This leads to a saturation of the receiving circuit, thereby preventing a proper detection operation.
To solve such a problem, there has been proposed a radar apparatus in which a transmission signal is prevented from being superimposed on a received signal (for example, refer to JP 09-243738 A). In the radar apparatus, a frequency modulation control signal is input to a voltage control oscillator, and a transmission signal that is frequency-modulated is supplied to a transmission and reception antenna via an antenna sharing unit to be transmitted toward a target object as a radio wave. The transmitted radio wave is reflected by the target object and is received by the transmission and reception antenna. A mixer mixes the received signal with the transmission signal that is output from the voltage control oscillator and outputs a beat signal. Since a method of calculating a distance to the target object based on the beat signal is similar to that of general FMCW radars, a detailed description thereof is omitted.
In the conventional radar apparatus, time-division transmission and reception is carried out to avoid the above-mentioned superimposition of the transmission signal on the received signal. A transmission-side switch is provided between the voltage control oscillator and the antenna sharing unit, and a reception-side switch is provided between the antenna sharing unit and the mixer. The transmission-side switch is controlled by a transmission and reception control oscillator. The reception-side switch is controlled, through an inverter, so as to be turned off when the transmission-side switch is turned on (at a time of transmission), and to be turned on when the transmission-side switch is turned off (at a time of reception).
A signal is received with a delay time of Td=2R/c (c indicates the speed of light) corresponding to a distance R to the target object, from the transmission pulse wave generated by the transmission-side switch. Practically, due to a function of the reception-side switch, a signal is not received in a signal-receiving period when the transmission-side switch is turned on, but received only in the delay time after the transmission-side switch is turned off. As described above, the target object is detected in a state where the transmission signal is prevented from being superimposed on the received signal.
However, in a case where a circuit is actually configured by using the above-mentioned conventional radar apparatus, a switching noise is generated due to an influence of charging and discharging in a switch element when the reception-side switch is switched on and off, and in particular, a received signal corresponding to a target object existing in a short distance is disturbed in the above-mentioned signal-receiving period, whereby the short-distance detection capability is lowered or short-distance detection is disabled.